In recent years a polyethylene terephthalate-based film has been widely used in various industrial fields because of its excellent mechanical and electrical properties. In particular, as a substrate for a magnetic recording material, particularly an audio tape, a video tape, and a floppy disc, the material exhibits quite excellent characteristics. It is no exaggeration to say that the polyethylene terephthalate-based film is superior to all other films in respect of cost and performance.
However, in view of a recent tendency that the magnetic recording material is increased in a recording density and reduced in film thickness, it has been desired to develop a new film much superior to polyethylene terephthalate. In the case of a video tape, for example, it has been required to further increase the recording time and thus it has been attempted to reduce the thickness of the base film. With recent standardization of an 8 mm video tape which may be recorded by video camera, it has been increasingly demanded to miniaturize the unit and densify the recordings for selling in the market. In order to satisfy such requirements, it is sufficient for the magnetic tape to have a high strength in a longitudinal direction, while a suitable strength in a transverse direction.
Since there has been no suitable film, a polyester film having a strengthened tensile strength in a longitudinal direction (hereinafter referred to as a "tensilized film") has been used. It has now been revealed, however, that such tensilized films have various disadvantages.
The tensilized polyester film is stretched by techniques such as (1) a method of stretching the film in a transverse direction followed by stretching it in a longitudinal direction to a greater extent than in the transverse direction; (2) a method of biaxially simultaneously stretching the film followed by further stretching it in a longitudinal direction; and (3) a method of stretching the film in a longitudinal direction to a greater extent followed by stretching it in a transverse direction but to a lesser extent; and the like. In accordance with any of the above conventional methods, stretching is difficult and, furthermore, because of breakage, for example, continuous operation is difficult to carry out. It is assumed that a method for the most preferable continuous production and suitable longitudinally stretching tensilized film are to stretch in only a longitudinal direction and to heat-set without stretching in a transverse direction. A uniaxially stretched polyester film produced by the above stretching method is liable to fibrillate to be torn in a longitudinal direction and, therefore, is unsuitable for a magnetic recording tape. In the case of a polyester film, since it is necessary to stretch the film in a transverse direction when the above method is employed, the strength thereof in a longitudinal direction cannot be increased to a great extent. On the other hand, if the longitudinal strength thereof is intended to increase, productivity is seriously lowered because of its breakage, for example.
In the case of the tensilized polyester film, productivity is lowered as a result of breakage, for example, in producing a high strength film. Moreover, the higher the strength in a longitudinal direction is increased, the larger the thermal shrinkage, thereby lowering the dimensional stability.
Particularly for the purpose of densifying the recordings, a method of vapor depositing metal powder directly on a film is used in place of the conventional method of coating iron oxide together with a binder. Accordingly, it is necessary that low molecule polymers, and oligomers must not be extruded to the film surface and the film should be heated during the vapor deposition process. In the case of a polyester film, however, precipitation of oligomers inevitably occurs, and during the polyester longitudinally stretching step, the heat-setting step, the vapor deposition step, and the cooling step after vapor deposition, and so forth, the oligomers are formed, thereby causing "drop-out". Thus, the resulting film is unsuitable as a magnetic recording tape.
As a base film for magnetic recording tape, such as a vapor deposited video film, which is required to densify the recording, it has been desired to develop a film which is of high strength and excellent in heat resistance and dimensional stability, has reduced in the amount of oligomers precipitated, and can be produced in a stabilized manner.
As regards a floppy disc, it is also required to develop increasing the recording density with the tendency of compact disc. In the case of a method in which magnetic characteristics are improved in a metal vapor deposition and vapor deposition for vertical magnetization, it is desired that heat resistance is to be increased. In the case of a method in which the number of tracks is increased, it is desired that dimensional stability is to be improved by controlling irreversible changes due to a thermal or humid expansion and humidity and temperature.
On the other hand, a stretched film of polyethylene terephthalate has heretofore been used as an insulating or dielectric layer of an electric article in a large quantity because of its excellent mechanical and electrical properties, heat resistance, chemical resistance, and so forth.
However, the stretched film of polyethylene terephthalate has various defects and has been desired to be improved. That is, when the film is used for insulation of a motor or as a covering of an electric wire, if it is exposed to high temperature atmosphere for a long period of time, it is physically and chemically deteriorated, resulting in a deterioration in mechanical properties and insulation properties of the film. Thus, it has been desired to develop an insulating film having a high maximum permissible temperature. In the case of insulation of a motor for a refrigerator, plugging due to extracted oligomers by a coolant is a major cause for trouble, and thus a film free from extracted oligomers has been greatly desired.
In the case that a polyethylene terephthalate film is used as a dielectric material for a condensor, the dielectric loss tangent goes up above the temperature of about 100.degree. C., which may be an upper limit of practically applicable temperature. This phenomenon is related with the glass transition temperature of the polyethylene terephthalate and depends on the characteristics of the material. Thus, a film which has a high glass transition temperature and is inexpensive is desired.
A polyethylene terephthalate film can be used as a flexible print circuit plate which is prepared by laminating a metallic layer on the film. This plate, however, has poor heat resistance and is not durable at temperatures above 260.degree. C. at which soldering is usually applied. Thus, soldering can be applied only in a specific temperature range as low as 230 to 240.degree. C. Accordingly, the plate has a disadvantage in that a soldered part is of low reliability. Under such circumstances, it has been desired to develop a film which has good heat resistance and is of good dimensional stability so as to be durable in a solder bath maintained at above 260.degree. C.
In place of a glass substrate for a liquid crystal display panel, an amorphous film such as of polyether sulfone and the like or a uniaxially stretched film of polyethylene terephthalate and the like is being used. These films, however, have various disadvantages and have been desired to be improved. An amorphous film of, for example, polyether sulfone is advantageous in view of extinction because it is amorphous, but its chemical resistance and solvent resistance are extremely poor. Accordingly, during the formation of the substrate, difficulties are encountered in processings such as lamination and adhesion. In the case of a uniaxially stretched polyester film, since it is difficult to completely orient in one direction, its extinction is not sufficiently satisfactory, and it is difficult to apply working because its heat resistance is poor. Moreover, if the film is exposed to an elevated temperature for a long time, oligomers come out and appear on the surface of the film, resulting in a reduction in transparency of the film. Under such circumstances, it has been desired to develop a film which has good solvent resistance, is not oriented, is excellent in heat resistance, and is free from precipitation of oligomers.
As described above, when the polyethylene terephthalate film, for example, is used as an insulating layer or dielectric layer, it is desired to solve the above problems so as to improve heat resistance and to prevent precipitation of oligomers and the like.
In order to satisfy the above requirements, several films have been porposed, including a polyimide-based film, an aromatic polyamide-based film, etc. These films, however, have disadvantages in high production costs, poor chemical resistance, and dimensional stability at varying temperatures and humidities. Accordingly, it is desired to develop a film which can be produced by melt extrusion, has crystallinity, and is superior to a polyethylene terephthalate in heat resistance, mechanical properties and dimensional stability.
Japanese Patent Publication No. 42167/73, for example, discloses polyethylene-2,6-naphthalene dicarboxylate which is one of polyesters. This polymer, however, has not yet been put into practical use because its characteristics are not markedly excellent and it is expensive.
The present inventors have made extensive investigations to develop an industrial material which is of high strength and excellent in heat resistance so as to satisfy the above described objects, using polyether ketone which has a secondary transition point, 80.degree. C. higher than that of polyethylene terephthalate, has a melting point, 100.degree. C. higher than that of polyethylene terephthalate, and, furthermore, exhibits high crystallinity. As a result, they have succeeded in overcoming difficulties considered in obtaining the desired good film. Thus, they have accomplished the present invention.